This invention relates to apparatus for the measurement of electric current, and other variables which can be related to, or expressed in terms of a current. While of general applications, the invention is of particular advantage in the measurement of very small current. For the purpose of this specification, measurement is to be taken as including the production of an output variable which is directly proportional to an input current whether or not that output variable is calibrated or indicated directly in current units. The invention can also be used for indication, measurement or control of variables in response to current changes, and to the establishment of currents of known or desired magnitudes.